This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners in the frequency range from below 10 MHz and up to 1 GHz.
Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models [5][6].
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested [1]; this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits.
Impedance tuners consist, in general, of a transmission line and a number of serial or parallel elements, fixed or adjustable, which create a variable reactance, allowing thus the synthesis of various impedances (or reflection factors) covering parts or the totality of the Smith chart (the normalized reflection factor area).
The relation between reflection factor and impedance is given by GAMMA, (Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50 Ohm.
At frequencies below a few hundred MHz impedance tuners based on the slide-screw principle [3] are impractical, since they must be at least ½ of wavelength (λ) long. At 100 MHz the electromagnetic propagation wavelength λ0 in air (or vacuum) being approximately 3 meters, it is obvious that such metallic structures, manufactured at high precision are very expensive and cumbersome to use in a laboratory. An alternative is to use a cascade of tuning sections based on variable capacitors and sections of transmission line [4].
The shortcoming of the tuners described in [4] is their limited bandwidth; as can be seen in FIGS. 2 and 3 the impedance coverage varies significantly from one frequency to another, especially because the sections of transmission line between capacitors are fixed, but also because variable capacitors have limited “dynamic range” or “maximum to minimum” capacitance values (Cmax/Cmin, typically 10:1 to 20:1); or capacitors with Cmax=100 pF have a minimum Cmin of around 5 to 10 pF, due to fringe field leakage and spurious elements. In traditional slide screw tuners it is the maximum value of the capacitance between probe (slug) and center conductor of the airline the limiting factor, since (a) the probe can travel horizontally [2] and (b) the minimum capacitance is virtually zero, or Cmax/Cmin close to Infinite. In the case of low frequency tuners there is enough capacitance (Cmax), but the minimum capacitance is not zero and phase between capacitors is not continuously adjustable, leading to situations that, whereas at certain frequencies there is a good coverage of the Smith chart (example f=2 GHz, FIG. 2), at other frequencies (example f=3 GHz, FIG. 3) there is a crowded area of impedances but also a lot of “empty” (or “un-reachable”) space, where the tuner cannot generate impedances.
To correct this shortcoming we propose tuners in which
a) the lengths of transmission line between capacitors,
and
b) the capacitors themselves can be exchanged in situ (i.e. without disassembling the unit), either manually or remotely. The new low frequency tuner comprises a cascade of at least three remotely adjustable variable capacitance blocks connected using exchangeable lengths of low loss transmission lines made using coaxial flexible or semi-rigid cables (FIGS. 4, 5, 9, 10, 13 and 14).